It is well known that graded refractive index rod lenses (GRIN) of one quarter nominal pitch length provide low insertion loss, fiber optic connectors by virtue of their property of expanding and collimating the light beam profile emanating from the core of an optical fiber. Such GRIN lenses are in widespread use and may be a light focusing glass device marketed by the Nippon Sheet Glass Co. and Nippon Electric Co. limited, Opaca, Japan under the trademark SELFOC. The SELFOC GRIN lenses are versatile, see, for example, the first listed pending U.S. patent application listed above.
The GRIN lenses create a mated connector assembly which is much less sensitive to translational errors (tolerances) which are otherwise due to imperfections in the alignment mechanism and of the connector body itself. Such connector assemblies are potentially useful for communications systems having small-core (i.e. 50 micrometer) optical fibers (small fibers are more sensitive to alignment errors).
Referring to FIG. 1, a mated pair of fiber optic connectors are depicted which employ GRIN rod lens collimators. A bushing 10 has a pair of ferrules, 11 and 12 receiving optical fibers 13 and 14. The fibers are potted in place at 15 and 16 to align with and match a pair of the GRIN lenses 17 and 18. An index matching fluid 19 is included to assure a somewhat more acceptable coupling between the two sets of optic fiber-GRIN connectors.
A hermetic, high pressure undersea penetrator also brings optical fibers and GRIN lenses together and is disclosed in the second referenced U.S. patent application above. Optical signals are transferred through a pressurized hull and, at least in principle, this fiber optic connector is capable of minimizing optical insertion loss attributed to optical fiber drawing imperfections and consequent core eccentricity within the optical fiber cladding. Hopefully, the physical location of the core of the optical fiber on the surface of the rod lens is adjusted at the time of connector fabrication and installation to provide optimum light beam collimation without resorting to mechanisms which must physically index to the outside of the fiber cladding.
Unfortunately, unless some type of measurement standard is relied upon during the adjustment process, connectors such as those shown in FIG. 1 and disclosed in the referenced related patent applications can only be manufactured in matched pairs; the location of the "receiving" fiber on the face of its corresponding rod lens must be adjusted in order to maximize the light capture from the "transmitting" fiber which is installed with a complimentary connector. In practice, a "good" connector is often employed as a quasistandard, and subsequent connectors are adjusted to mate with it, in the hope that the resulting connectors will efficiently mate with one another. Besides the obvious problem of defining what is a "good" connector, to serve as a standard, a practical difficulty arises when multiple standards are required so that interchangeable connectors can be installed by a variety of users for a variety of purposes. Due to unavoidable errors in defining and maintaining a repeatable standard, the realized connector performance is degraded with respect to the true potential of the design when referred to a matched connector pair. Even matched pairs will, in general, be sensitive to rotational motion within the alignment sleeve because no effort is originally made to adjust the axis of the collimated light beam so that it is exactly parallel to the connector ferrule, by which the connector is mechanically referenced to the outside world.
Clearly, a reproducible standard, definable by physical qualities such as linear measurements and angles is required to maximize connector performance and ensure interchangeability by assuring optimum fiber placement during the alignment process at the time of connector installation. The proper alignment technique therefore, must guarantee that each connector emit a properly collimated light beam which is, for all practical purposes, parallel to the connector ferrule.
Thus, a continuing need exists in the state of the art for a method of fabricating an optically aligned coupling between an optical fiber and a connector which is reproducible and capable of being installed by different users under a variety of conditions to provide optimum signal transfer.